"The fan is useful in December."
Translation:O ventilador é útil em dezembro.
while both verbs mean "to be", they are used in different contexts. lots of the time it has to do with how temporary something is, so if you're describing someone's emotions (which come and go) you'd say "ele está bravo" ou "ele está com raiva". if you're describing a more permanent attribute you'd say "ele é alto" ou "ele é legal". this rule doesn't work for all scenarios though, since you can say "a igreja está lá" (the church is there) which you would think isn't as temporary as an emotion but still warrants the use of estar (está) rather than ser (é)
maybe some chart exists that tells you in what circumstances to use each verb. take the following advice with a grain of salt because i'm not sure of the extent to which spanish and portuguese are similar, but the same phenomenon (ser and estar) is present in the spanish language. since more learning materials are available online for spanish than for portuguese, maybe you could look for a ser y estar chart on google and it'll tell you more specifically when to use ser and when to use estar. once again, spanish and portuguese, though similar, vary greatly, and the rules for the use of ser and estar may vary slightly. i can tell you that the aforementioned sort-of rule about how temporary something is also applies to spanish "él es alto" (he is tall), "él está enojado" (he is angry).